Get extra disconnnects, and clamps.
One of my most useful tools is a bit of tubing with a bike tire valve on one end and a gas QD on the other. With that I can use a bike hand pump to push line cleaner, rinse water, even Star san through the lines without wasting CO2. I suppose in a pinch I could also use it as a party pump, but I haven’t.
Also, They’re a bit pricy for what they do, but I’ve really come to like these keg level indicators. Basically there’s a little orange ball that rides down the side of the keg as you empty it. The probably don’t work if you dry-hop in the keg though.
Extra kegs are good too. I do both 5 gallon extract and 3 gallon all grain, so I have a few kegs of each size.